Dual GPU or SLI
 

Which is better or will give better performance? Dual GPU cards or SLI with two seperate GPU'S

SLI with two separate cards, dual GPU cards are usually downclocked to keep within a power TDP envelop.

That's true but the price is usually far better on the dual gpu card.

Eg GTX 590 $700
2* GTX 580 $900 - $1000

2 580s were a bit faster but not by a lot. to get the same price it'd be 2 570s which were not faster.

I also wonder, given the reduced power consumption of the 680 if they might be able to dual it and still fit that within two 8 pin connectors with no downclocking this time.

Isn't a 590 essentially 2 570's?

no, is 2 x 580 downclocked chips. Downclocking is done because of the constraints of operating under simple air cooling and taking only 2 slots. Fancier solutions (the triple slot Asus MARS II) or water cooled blocks can take care of that and simply overclock the chips. Power intake has never been a problem for any card : if you need a more powerful PSU you will have thought of that before getting the videocard.

Exactly what Supa said